Any emergency that involves a severe injury or death is a critical incident. This incident could be amplified should the emergency involve a family member or friend. The stress that these incidents cause may overwhelm a first aider and shut down their ability to cope. This is what is known as critical incident stress (CIS). This condition may have a great impact on the first aider suffering from it, and if left un-treated, this stress may lead to a more serious condition known as post-traumatic stress syndrome.